Background & Aims: The proper concentration of fluoride in drinking water can prevent dental caries and the concentration of above the standard causes dental fluorosis. The aim of this study was to determine the relationship between fluoride concentration in the drinking water with DMFT index and dental fluorosis in 11-12 years old students in rural areas of Oshnavieh.Materials & Methods: In this descriptive-cross sectional study, all 11-12 years old students were selected from 42 villages with a population of more than 300 people, in the city of Oshnavieh. The study population included 436 and 478 female and male students, respectively. The duration of the study was 5 months and study population was examined according to the WHO guidelines. The rate of dental caries and fluorosis in students in each village was determined by DMFT index and based on Dean Classification, respectively. The mean fluoride concentration was determined based on the 10-year results of the standard tests of the rural water and Sewage Company. The relationship between fluoride concentration in the drinking water, DMFT index and dental fluorosis were analyzed by linear regression analysis using SPSS software.Results: The mean of fluoride concentration in drinking water was 0.2 ± 0.09 mg/L. The mean of DMFT index in the studied students was 3.11±1.48 (female students 3.06±1.57 and male students 3.16 ±1.68). No dental fluorosis was detected at any level in any of the examined students. The results of statistical analysis showed that there is a significant reverse relationship between fluoride concentration and DMFT index (R2=0.58, P valve=0.001).Conclusion: The results of this study showed that, for one unit increasing of fluoride, the DMFT index 3.74 unit increased. Also, relationship between fluoride and DMFT index is significant and inverted.